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Abstract

Infestations of the exotic perennial Centaurea maculosa Lam. (spotted knapweed) hinder the restoration and management of native ecosystems on droughty, infertile sites throughout the Midwestern United States. We studied the effects of annual burning on knapweed persistence on degraded, knapweed-infested gravel-mine spoils in western Michigan. Our experiment included 48, 4-m2 plots seeded to native warm-season grasses in 1999 using a factorial arrangement of initial herbicide and fertility treatments. Beginning in 2003, we incorporated fire as an additional factor and burned half of the plots in late April or May for three years (2003-2005). Burning increased the dominance of warm-season grasses and decreased both biomass and dominance of knapweed in most years. Burning reduced adult knapweed densities in all three years of the study, reduced seedling densities in the first two years, and reduced juvenile densities in the last two years. Knapweed density and biomass also declined on the unburned plots through time, suggesting that warm-season grasses may effectively compete with knapweed even in the absence of fire. By the end of the study, mean adult knapweed densities on both burned (0.4 m-2) and unburned plots (1.3 m-2) were reduced to levels where the seeded grasses should persist with normal management, including the use of prescribed fire. These results support the use of carefully timed burning to help establish and maintain fire-adapted native plant communities on knapweed-infested sites in the Midwest by substantially reducing knapweed density, biomass, and seedling recruitment, and by further shifting the competitive balance toward native warm-season grasses.